Ozone layer

Ozone is a form of oxygen whose molecules contain three oxygen atoms. It is found throughout the atmosphere, but most atmospheric ozone is concentrated in the stratosphere, roughly 15–30 kilometres above Earth's surface. There, ozone is continually formed and broken apart through reactions involving sunlight and oxygen. This natural cycle absorbs much of the Sun's ultraviolet radiation, reducing the amount of harmful UV reaching the surface. Ozone near the ground is a pollutant; the protective ozone layer refers to stratospheric ozone.

From the 1970s, scientists established that human-produced compounds containing chlorine and bromine could deplete stratospheric ozone. When these long-lived compounds reach the stratosphere, ultraviolet light releases reactive halogen atoms that participate in catalytic reactions, allowing an atom to destroy many ozone molecules. The most dramatic recurring depletion occurs over Antarctica in Southern Hemisphere spring. Very cold winter conditions form polar stratospheric clouds, while the polar vortex isolates the air; when sunlight returns, chemical reactions rapidly destroy ozone. The term ‘ozone hole’ describes a broad region of unusually low ozone, not a literal opening in the atmosphere. Ozone levels also decline to varying degrees outside Antarctica.

The Montreal Protocol, signed in 1987 and in force from 1989, established a global schedule to phase out the production and consumption of controlled ozone-depleting substances. The agreement has been adjusted as scientific evidence and technology have advanced. Because many of the chemicals persist in the atmosphere for years or decades, reducing emissions does not restore ozone immediately. A 2022 international scientific assessment, summarized by the World Meteorological Organization in 2023, reported that the phase-out of nearly 99 percent of banned ozone-depleting substances had contributed to recovery. If current policies remain in place, that assessment projected a return to 1980 ozone levels around 2040 for most of the world, 2045 in the Arctic and 2066 over Antarctica. These are projections, and natural variability affects year-to-year ozone measurements.